1. Field of the Invention
The present invention relates generally to multilayer capacitors, and more particularly, to multilayer capacitors having an improvement in the structure of their inner electrodes.
2. Description of the Prior Art
An example of a conventional multilayer capacitor will be described with reference to FIGS. 16A to 17B. This multilayer capacitor is constructed using a sintered body 5 obtained by alternately laminating ceramic green sheets 1 each composed of dielectric ceramics having an inner electrode 2 formed on one surface and ceramic green sheets 3 each composed of dielectric ceramics having an inner electrode 4 formed on one surface, as shown in FIGS. 16A and 16B, pressing the laminated body thus obtained in the direction of thickness and then, cofiring the same.
More specifically, in the sintered body 5, the above inner electrodes 2 and 4 are laminated while being separated by dielectric layers, as shown in FIGS. 17A and 17B. In addition, outer electrodes 6 and 7 are formed on both end surfaces of the sintered body 5. Capacitance is formed between the inner electrodes 2 and 4 which are overlapped with each other while being separated by the dielectric layer.
The inner electrodes 2 and 4 are made of a noble metal such as Pd or an Ag-Pd alloy. Consequently, the above described multilayer capacitor has the disadvantage of being high in fabricating cost because the area of each of the inner electrodes 2 and 4 is large and the inner electrodes 2 and 4 are made of a high-cost noble metal material.
As disclosed in Japanese Patent Laid-Open Gazette No. 156619/1990, a multilayer capacitor using inner electrodes each having a plurality of inner electrode fingers has been proposed. The multilayer capacitor of this type will be described in accordance with FIGS. 18A to 19. In this multilayer capacitor, a ceramic green sheet 11 having an inner electrode 12 having a plurality of inner electrode fingers 12a printed thereon and a ceramic green sheet 13 having an inner electrode 14 having a plurality of inner electrode fingers 14a printed thereon are used, and the plurality of inner electrode fingers 12a and 14a are formed so as to have the same width and to be respectively arranged while being separated by gaps 15 and 16 having the same width.
In the multilayer capacitor using the above described inner electrodes 12 and 14, the plurality of inner electrode fingers 12a and 14a are arranged so as to be overlapped with each other while being separated by dielectric layers 5a, as shown in a cross sectional view of FIG. 19. In this structure, an electric field is concentrated at the edges each of the inner electrode fingers 12a and 14a so that a phenomenon referred to as the so-called edge effect occurs, thereby making it possible to increase the capacitance by 20 to 30%, as compared with the above described multilayer capacitor using the inner electrodes 2 and 4 having the same area. Accordingly, although both structures obtain the same given capacitance, the area of each of the inner electrodes can be decreased in the multilayer capacitor having the structure shown in FIGS. 18A to 19, as compared with the multilayer capacitor shown in FIGS. 17A and 17B. Consequently, the multilayer capacitor shown in FIG. 19 can be lowered in fabricating cost.
However, disadvantageously, the inner electrodes 12 and 14 respectively have the plurality of inner electrode fingers 12a and 14a. Accordingly, in respectively forming the inner electrodes 12 and 14 on the ceramic green sheets 11 and 13 or in laminating the ceramic green sheets 11 and 13, if the inner electrodes 12 and 14 which are overlapped with each other in the vertical direction are shifted in the lateral direction (shifted in the direction represented by arrow P in FIG. 19), a portion appears where the inner electrode fingers 12a and 14a are not overlapped with each other in the direction of thickness. Consequently, the multilayer capacitor shown in FIG. 19 has the disadvantage of varying in capacitance very largely as represented by a solid line C in FIG. 7 depending on the amount of the above described shift between the inner electrodes in the lateral direction.
Another multilayer capacitor using inner electrodes 32 and 34 shown in FIGS. 20A and 20B has been proposed so as to reduce the above described variation in capacitance. In this multilayer capacitor, the inner electrode 32 has a plurality of inner electrode fingers 32a, and the other inner electrode 34 is formed in a rectangular shape having no inner electrode fingers. The inner electrodes 32 and 34 are alternately laminated, to constitute a multilayer capacitor shown in a schematic cross sectional view of FIG. 21.
As obvious from FIG. 21, the plurality of inner electrode fingers 32a are overlapped with the inner electrode 34 uniformly formed in the lateral direction while being separated by a dielectric layer 5a. Consequently, even if the inner electrode 32 having the plurality of inner electrode fingers 32a and the inner electrode 34 are shifted in the lateral direction, that is, in the direction represented by an arrow P in FIG. 21, the variation in capacitance is reduced. In the multilayer capacitor shown in FIG. 21, however, the inner electrode 34 is printed over a large area so as to have a rectangular shape, as shown in FIG. 20B. Consequently, the multilayer capacitor shown in FIG. 21 has the disadvantage of being high in cost. Moreover, it is also found that the multilayer capacitor shown in FIG. 21 has the disadvantage of being lowered in breakdown voltage.